vla: enable sizeof operator to work with variable length arrays

vla: enable sizeof operator to work with variable length arrays

In C99 the sizeof operator computes the size of a variable length array
at runtime (6.5.3.4 The sizeof operator). This patch reflects the semantic
change in the debugger.

We now are able to get the size of a vla:

1| void foo (size_t n) {
2|   int vla[n];
3| }

(gdb) p sizeof(vla)

yields N * sizeof(int).

gdb/ChangeLog:

	* eval.c (evaluate_subexp_for_sizeof) <OP_VAR_VALUE>: If the type
	passed to sizeof is dynamic evaluate the argument to compute the length.